Since developing, charging and transfer rolls used in a laser printer or the like are in contact with a photoreceptor because of its mechanism, in addition to its semi-conductivity, low hardness, satisfactory compression set and shape retentivity to the ambient environment are required. Commonly, an ethylene-propylene rubber, a urethane rubber, an epichlorohydrin rubber, an acrylonitrile-butadiene rubber and the like have been used in the form of a blend or used alone in response to volume resistivity required to each of conductive, developing and transfer rolls.
There has hitherto been employed, as a method of realizing low hardness of a rubber material, a method of adding a low-viscosity substance such as a process oil or a plasticizer. However, the addition of these plasticizers to a conductive rubber composition is not preferred since there is a fear of contamination on a photoreceptor. Therefore, the addition of a liquid acrylonitrile-butadiene rubber with a low molecular weight has been studied so as to obtain a low-hardness rubber material without causing contamination of the photoreceptor (see Patent Document 1). The addition of the liquid acrylonitrile-butadiene rubber causes adhesion onto the photoreceptor and exerts an adverse influence on compression set. This reason is considered that a low-viscosity liquid rubber is not involved in crosslinking.